Take my best regards. A client (X) was interviewing me for a corporate video. We discussed in details about the video. After some days of interviewing, she offered me for the job! That's cool. But there is a problem! I didn't get the offer from her (X)!! Today, a different client (Y) from a different account invited me for a new job. After accepting that invitation, the client (Y) directly offerd me for the job before I asked her about the details of the job! And the client (X) who was interviewing me from 3 or 4 days, she messaged me "we're going to hire you with the account of (Y) to card problems". I never have faced this kind of situation here. I am confused that should I go for this job though both clients has good investments and review?

I would like to confirm that it's possible that a client is working with a team mate who has another account on Upwork, and they are working together on this project. This is most likely why your first client (X) informed you that you will be hired through client (y)'s account.